Pixelation on HD viewing and recording

#1
At least once per hour when watching or recording HD channels I get very bad pixelation and break up of the picture. When it's really bad, the box reboots itself.

I did have the custom firmware installed, so yesterday I manually installed the official firmware, then I reformatted the HDD, ran the HDD test (no errors reported), factory reset and retuned, making sure that the channels tuned are what digitaltv.co.uk says they should be for my post code.

The problems recurred within 2 hours of all of this.

Signal strength is always between 75 and 80%, and quality always reported as 100%

Upstairs I have a freeview HD TV which has never pixelated, so I do not think that this can be blamed on my aerial or coax in the house.

Has anybody else seen similar problems? Should I be asking HUMAX for a replacement?
 

MartinLiddle

Super Moderator
Staff member
#3
Signal strength is always between 75 and 80%, and quality always reported as 100%
Is the quality 100% when pixellation is occurring?
Upstairs I have a freeview HD TV which has never pixelated, so I do not think that this can be blamed on my aerial or coax in the house.
Have you tried the Humax upstairs to rule out the cabling to downstairs?
 
OP
OP
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DRY411S

New Member
#4
Thank you for the reply.

Is the quality 100% when pixellation is occurring?
I will check next time I'm watching HD and report back.

Have you tried the Humax upstairs to rule out the cabling to downstairs?
Not yet, is the final item on my checklist of things to try.

Based on your questions, do you suspect that there may be a coax/aerial problem?
 
#6
I had some recent problems like this (poor quality, low signal at random but happening every few minutes). Plugging the coax into the TV (bypassing the hummy loop-through) and seeing the same problem there ruled out it being a hummy specific problem. After much prodding of the coax, it seems my signal strength (and quality) varies wildly as I wiggle the few feet of loose cable between where it comes into the house and the AV gadget pile; solution was simply to immobilize the cable better, in a good configuration. Still not entirely happy with the result; signal strength definitely still not what it used to be. Have an idea this all started with some windy & wet weather but then it doesn't make much sense it's moving the indoor cable which has so much effect.
 

Black Hole

May contain traces of nut
#7
It's possible. The aerial coax is not just another piece of wire, it's a transmission line. Any poor connection in the cable run (even a kink in the cable) creates a point at which some of the signal passes on but some of it gets reflected back up the cable.

If you have two such reflective points in the cable, either constructive or destructive interference can build up if the distance between them is just right compared with the wavelength of the signal in the coax (which is a few tens of centimetres). It is equivalent to sloshing water back and fore in a bath at just the right frequency to build up a standing wave (which is an example of constructive interference).

If these are the circumstances, the multiplexes suffering signal loss due to destructive interference will be very selective (but it should affect all services on one multiplex). It will only occur where the cable distance between the two mismatch points is an exact number of wavelengths plus about half a wavelength (or perhaps that should be quarter of a wavelength - I'm a bit rusty). So if you already have a mismatch (weather induced) there might not be a problem, but then adding another one further down creates the conditions for destructive interference. I actually used this trick to eliminate some unwanted side-lobe pick-up.

Alternatively it might simply be a case of signal loss at the top end being not too bad, but then further signal loss at the bottom end taking it over the digital cliff.
 

Ezra Pound

Well-Known Member
#8
it seems my signal strength (and quality) varies wildly as I wiggle the few feet of loose cable between where it comes into the house and the AV gadget pile
Sounds like it could be RF noise being induced into the co-ax which is eminating from the 'AV gadget pile', you could try re-terminating the aerial plug that is on the end of the co-ax or improving the sheilding of the co-ax (not easy without replacing it). Also keeping the co-ax away from any mains cables as much as possible may help
 
#10
Took a closer look at the signal strengths on the hummy "manual tune" page, by channel:
21: 36/100 signal strength/quality (this is the HD one)​
24: 35/100​
27: 46/100​
39: 33/100​
42: 28/100​
45: 10/40 (pixelation! sound glitches!) though arguably not much loss; this one carries a total bunch of trash. (OK, plus E4+1, Dave, and Really does have those strangely compelling "how stuff is made" programmes...).​
The TV's own tuner agrees, also reporting weak/poor for 45 and strong/good for the rest (the SD ones anyway; it's not an HD telly).

Can anything be diagnosed/eliminated from one particular channel being weak? We got a wideband aerial years ago when freeview first appeared and reception has always been 100% quality on all channels previously. Although like Black Hole says, if you can't really consider any part of the aerial/cable in isolation, maybe it's just something about that frequency...
 

Ezra Pound

Well-Known Member
#11
Can anything be diagnosed/eliminated from one particular channel being weak?
Is the Humax tuned into two different transmitters? I ask because 21, 24 and 27 are in Group A and 39, 42 and 45 are in Group B these will be covered by a widband aerial but I would have though Muxs on a single transmitter would be closer together, have a look in the 'Nework' column

channels.jpg
 
#12
Ezra - the work I've been doing over the last 2 days shows the webif (at least on my system) power and Q data is not updated - sometimes for
hours at a time - it bears no relation to the values measured on the box and seems to have historical use only.
I'd suggest anyone checking levels to always use the box itself (unless I have yet another problem and it should be "live" !)
It would be good if it could be real time as the box will keep dropping out. (bad design at humax - actually bad design philosophy but thats another story)
 

af123

Administrator
Staff member
#13
Ezra - the work I've been doing over the last 2 days shows the webif (at least on my system) power and Q data is not updated - sometimes for
hours at a time
It's only a snapshot from the last time you did a channel search.
 
#17
Is the Humax tuned into two different transmitters?
I did wonder about that. We get our signal from Craigkelly which according to http://www.tvretune.co.uk/
has Groups 1-5+HD on channels 27,24,42,45,39,21 (Group 4/Channel 45 being the problem weak signal).

The only other viable transmitter in the area I know of is Black Hill
(and in fact a small minority of aerials on the roof point that way; I think we actually used it too before we got the digital/wideband aerial installed);
that has Groups 1-5+HD on 46,43,41,44,47,40 so no overlap.

In the past we have had such problems with previous auto-tuning freeview kit picking up duplicate channels that I'm now in the habit of just working through the Craigkelly list just tuning those frequencies. Not sure what the situation actually is post "switchover", but I've stuck with the "manual" habit. Same for the TV.

Hmmm... this just gave me the idea of checking the Black Hill frequencies on the tuners too. None of them show up any signal except, curiously, Ch 46, which actually reports better reception on the Hummy (10%/40% strength/quality) than Craigkelly's Ch 45 (currently 10%/10%)! (Neither shows at a quality worth anything though). TV reports a similar story. All very strange.

Did go up on the roof today for a look to see if anything was obviously wrong there. It's conceivable the cable has been damaged by thrashing about in wind
where it goes over the edge of the roof (but not easily/safely closer inspectable), and the box on the aerial the coax goes into was slightly unclipped,
making me think rain might have got in (vaguely remember the guy who installed it saying the main reason aerial installations go bad isn't them
blowing down but water getting into the coax). Generally wiggled and tightened things up but no improvement.
 
#18
Some things I've found that cause issues in case this helps:
1. Strip lights
2. Aeroplanes (doubly so if its raining)
3. central heating boilers
4. Broadcasters messing about and failures (last weekend almost killed all HD channels - 304 went off air completely for a while)
5. Relying on "professional aerial installers"
6. Plugging other things into the same power extension as the boxes
7. I have just discovered the Humax doesnt like 3 or more USB drives installed even if they are self powered - eventually things start to play up
Not confirmed yet but I think so.

In the last few years :
I have had 3 aerials and installers - 4 re-visits to check - 2 complete coax runs - 5 Freeview boxes of various makes - dozens of attemps at various amps and settings as advised
by said installers etc. Nothing helped. I even ended up taking the wall sockets appart and installing filters.
I had routine pixelation and breakups except during holidays and and national events such as jubilee and royal wedding
during which reception is always perfect.
.
For the last 7 days (appart from sat-mon which I suspect was hardware problems in sutton - see 4 above) I have had clean reception
and power levels of 80-95% and Q of 100% STABLE as measured on the box (much pixelation seems to be rapid drops in Q level)

To get this I am using a set top amplified log periodic aerial with a seperate amp at the TV end sharing the boost between them.
Careful adjusting was needed to prevent overdriving - especially at night when levels tend to increase. A compas to check the
the direction also helped.

So far this has cured everything except a once in a while small pixelation when the strip light downstairs is switched on. This I'll sort later.
 

Black Hole

May contain traces of nut
#19
If you are getting RF interference from a florescent lamp everything points to an extremely weak signal or a lousy signal path - and yet you seem to have eliminated those possibilities.
 
#20
Yes you are right BH- I thought that myself for a long time.
It may just be the strip light needs a new starter or something (I dont know much about them) but I'm
letting the current config settle now before trying anything else.
So far so good though - I've never had it so stable before.
I'm wondering if a rooftop log periodic would be better around here - maybe thats what did it.
(The last time I tried this set top aerial they were still on low power)
 
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